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b=WISsPPXZP8H+KWyPmwNcqB2cLI2ZXJEgwzx3oexLfVxNWsyzmmXvN4X8DRy2Vd+0I EbneXdraYBYE+MOoiR/pSsF+nWnDojcHw4a550rxBMavXoTTYjVpbThn2Esb1/mPa1 ftTV+KJ2N0MN9DcXW9DRciHxSUVosh+k6tNP9jgVW7Ubnbifd096X4PfwCv2j6mhSb g7crbmDfyxCsEpRNoZ284TSxRaTLFPLQZENYhzOhI6bIQAAacSRaLhEQ2DFqzzFLYq swu2jtmpyrg+ODxhunThg21TqJvN6KmpYidVnI9akO+895qOLt3lG+1pA5Zt1norMc HXH6UxWptzoCQ== Message-ID: <30f58666-a798-4028-a2c3-9eb63612c4cc@kernel.org> Date: Fri, 25 Sep 2026 09:06:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v6 3/3] mm: implement page refcount locking via dedicated bit To: Ilya Gladyshev , Zi Yan , Linus Torvalds Cc: akpm@linux-foundation.org, andrew+netdev@lunn.ch, apopple@nvidia.com, artem.kuzin@huawei.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, edumazet@google.com, harry.yoo@oracle.com, hramamurthy@google.com, ivgorbunov@me.com, joshwash@google.com, kirill@shutemov.name, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, muchun.song@linux.dev, pfalcato@suse.de, rppt@kernel.org, surenb@google.com, vbabka@suse.cz, willy@infradead.org, yuzhao@google.com References: <56e036665dbeb0a8f51d697220d5ed84f210994c@linux.dev> <570cf649-a8bf-4073-be71-207fbb2bef40@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam08 X-Rspamd-Queue-Id: CD1B3160008 X-Rspam-User: X-Stat-Signature: 1m84jsgp4n639kc6t9o1p4t6s11ymar7 X-HE-Tag: 1790319992-650447 X-HE-Meta: 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 c8Htn56r UvsqFJ2jCY2rTsSrdRQvUyH2M+nT+umIMqa3wo23xBc6mK5r8iDuVTN6DvBIRUrJwP8+KzDFYRIpph3nVZb0K2hKKYtzkAZhM/LJ0OSX19buORmYGynNWpEsrdwxHeuIlSmioJnhlV0DMhETpqIDSPuIIgKMIJYc99A/QD7t83YRYfjSg52ejJIHjlY/K2iWc/Xxzw9N4SMjaAz/CW1C+xcXfy4E9CZaFjtzbFOR73umZe7qa9faZGs5XLaphc7ZFF7t8NUZcOOhWst15ltOPxJMQMzQmAN4/sYofuhyuWme96funZKuBD+1pqn2uhV6gl5hpc+edq2H9Z9xSwIafXYWyq5ZLhXVQLyVEy/8TT0yVPMBQRGZvpc7lflcXbqyxlkGkDphctMnJcv2HilDaEv4m7Q== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 9/25/26 09:03, Ilya Gladyshev wrote: > September 25, 2026 at 5:44 AM, Zi Yan wrote: >> >> On Thu Sep 24, 2026 at 4:06 PM EDT, Linus Torvalds wrote: >> >>> >>> >>> I think this all needs to be entirely generic to all pages. Agreed. >>> Everybody needs to do it right. >>> >> It assumes all callers of page_ref_dec_and_test() are free page >> functions, but there are two exceptions: >> >> 1. compaction_free() uses folio_put_testzero(), which calls >> page_ref_dec_and_test() to put not used migration dst page back to >> isolated freelist instead of freeing them. >> >> 2. virtio_mem_fake_offline_going_offline() drops the last reference of >> fake-offline pages, so that they can be offline. >> >> They will need to be converted to not use page_ref_dec_and_test() in >> addition to a common page freeing function. And any such future use >> needs to be banned. >> >>> >>> But I think that in the scenario that Ilya mentioned: >>> >>> Refcount operations as follows (FR = FROZEN): >>> >>> A: DEC 1 -> 0 [ page_ref_dec_and_test() ] >>> B: INC 0 -> 1 [ folio_try_get() ] => success >>> B: DEC 1 -> 0 [ page_ref_dec_and_test() ] >>> B: CAS 0 -> FR [ page_ref_dec_and_test() ] >>> B: * deallocates via __folio_put() * >>> >>> C: * re-allocates page * >>> C: DEC 1 -> 0 [ page_ref_dec_and_test() ] >>> A: CAS 0 -> FR [ page_ref_dec_and_test() ] => success >>> A: * deallocates directly via free_frozen_pages() * => BUG >>> >>> the big here is that the de-allocation was done not from the state of >>> the *page*, but from the state of who finished it off. >>> >>> So that >>> >>> A: CAS 0 -> FR [ page_ref_dec_and_test() ] => success >>> >>> is not a problem per se. It's a fine case of "we had one single unique >>> final free". >>> >>> The failure is that A then goes on to do that >>> >>> "deallocates directly via free_frozen_pages" >>> >>> because A mis-judged the page as having the old stale state that it >>> had when *A* was using it. >>> >>> IOW, I think the soluition is either: >>> >>> (a) every user of put_page_testzero(page) agrees tro do exactly the >>> same thing and there is no difference between how that final >>> "0->FROZEN" is dealt with, so it doesn't matter that A is freeing a >>> page that C allocated for something else >>> >>> or >>> >>> (b) the CAS 0 -> FR state sequence always looks at the *page* to >>> decide what to do, never at the caller state (ie A will look at >>> whatever C wrote when it allocated the page to know how to do that >>> final free). >>> >>> Hmm? >>> >> Just wonder if we could make 1 -> FROZEN atomic to get rid of the gap. > > Hmmm, I’m afraid that since you need CAS for a safe 1->FR transition, > it will result in a CAS loop for the decrement itself (like in > atomic_sub_unless). I was playing with exact this idea when I was reviewing one of the earlier versions; didn't manage to make it fly in a nice way :) -- Cheers, David